The present invention relates generally to a method of lubricating contacting, sliding surfaces and, more particularly, to lubricating contacting and rubbing surfaces exemplified in microelectromechanical (MEMS) systems using a vapor-phase operating environment.
Commercially successful MEMS devices to date have either nonmoving parts or contacts whose lateral motion is very restricted. Many more exciting applications can be attained with MEMS devices consisting of moving, touching, and rubbing structures. These include gears and motors that can enable much more complicated mechanical functions at the micro- and nano-scale. However, the effects of adhesion, friction and wear of MEMS devices are challenging the development and commercialization of more sophisticated micromachines.
The tribological behavior of contacts in MEMS technologies differs from those in macroscopic engineering structures. At the macroscopic scale, millions of asperities give rise to the parametric relationships that we are familiar with, such as Amanton's law which depicts the friction coefficient to be independent of contact area and applied load. In MEMS, real mechanical contacts typically consist of a few nanometer-scale asperities that touch. At these small scales, Amonton's law breaks down and individual asperity contact behavior must be considered. Additionally, forces that are negligible at the macro-scale become significant at the microscopic length scale and smaller. These include electrostatic or van der Waals forces between contacting and non-contacting surfaces, and capillary forces due to liquid menisci. In the macroscopic scale, gravity dominates over adhesion. However, in the micro- and nano-scale, gravitational and inertial forces are negligible and adhesion becomes significant. It is important to point out that in some cases the magnitude of these forces is comparable to the actuation forces that can be provided with on-chip actuators.
Adhesion, friction and wear have been the greatest limitations to the development of MEMS devices that rely on contacting and rubbing surfaces. These tribological problems associated with micromachines cannot be solved by applying conventional lubricants utilized in the macro-scale, such as liquid lubricants. In micromachines, the viscosity of liquid lubricants causes severe power dissipation problems and causes devices to move slowly, negating one of the principal advantages of micromachines, i.e. low inertia that enables rapid mechanical switching.
Coupling agents having a functional end that can bond with the surface and a low energy end group pointing toward the free surface have been very successful as processing aides. These coatings can be applied from the liquid phase immediately after removal of the sacrificial layer to prevent adhesion as liquid dries from the part, or from the vapor phase once the liquid remaining after the release etch is supercritically extracted. In either case, the coupling agents form a chemisorbed monolayer of organic molecules that is ideally densely packed on the surface, so that the low energy end groups cover the entire surface. These surfaces are hydrophobic and resist the adsorption of water as well as organic contaminants much better than uncoated devices do. These types of coatings have been critical to having operating devices survive packaging operations and remain functional. However, it is well recognized that since these coatings are chemically bound to the surface and only a monolayer thick, that once they are displaced by wear or reaction with atmospheric species they leave the surface unprotected. Work over the last decade with various MEMS structures having sliding surfaces has shown that chemisorbed monolayers alone are insufficient to allow long operating life. Long-life MEMS devices with sliding surfaces will require a system for replenishing the lubricating layer as the device operates.
No commercial MEMS devices with repetitive sliding contacts exist today. MEMS devices are typically fabricated with silicon-based materials because mature lithographic fabrication techniques are available from the semiconductor industry; however, silicon and its native oxide exhibit high friction and poor wear resistance in sliding contacts. To overcome these problems, various coatings have been developed and extensively studied, but with questionable reliability and feasibility. Thin protective coatings are subject to wear during operation, limiting device lifetime. A means of continuously replenishing the protective layer on these surfaces is required to overcome this limitation. An additional requirement is that the lubricant must be deposited conformal to the surfaces, not possible with typical physical vapor deposition (PVD) techniques which deposit via line-of-site or chemical vapor deposition (CVD) techniques which deposit non-uniformly and do not easily coat buried surfaces. In vapor-phase lubrication (VPL), the adsorption of gas-phase molecules on device surfaces can produce a conformal lubricant layer. During sliding contact, adsorbed molecules can be desorbed from the surface; however, as long as the vapor pressure of lubricating molecules is maintained, these molecules immediately re-adsorb and replenish the lubricant film. Adsorbed films have been used to repassivate dynamic digital mirror arrays; but this device does not have deliberate sliding contacts. Previous VPL required elevated temperatures and lubricant molecule activation via precursor catalytic metal coatings. Useful would be a vapor-phase lubrication strategy that works in ambient conditions without special coatings.